Optical measurement systems may be used to measure various parameters or characteristics of a specimen (e.g., a device-under-test (DUT) or sample material or component). Generally, an optical measurement system directs incident light at the specimen, and the specimen may produce polarized or unpolarized reflected light, polarized or unpolarized transmitted light, and electrical signal (e.g., current and voltage) in response to the stimulus. The optical measurement system typically includes devices to detect and analyze the reflected light, transmitted light, and/or electrical signal to measure the desired parameters or characteristics of the specimen.
As an example, an optical measurement system for use in measuring the extrinsic quantum efficiency (EQE) of a specimen may include a light source (and other associated components) to generate and direct a defined incident light at the specimen. Such optical measurement system may also include a reference detector to detect a portion of the incident light, and an electrical detector to measure an electrical response (e.g., current or voltage) generated by the specimen in response to the stimuli. Such optical measurement system may include an analysis component to calculate the EQE of the specimen based on signals generated by the measurement system.
Similarly, as another example, an optical measurement system for use in measuring the intrinsic quantum efficiency (IQE) may include a light source (and other associated components) to generate and direct a defined incident light at the specimen. Such optical measurement system may further include a reference detector to detect a portion of the incident light, a specular reflectance detector to detect light reflected at an angle from the specimen, a diffusive reflectance detector to detect scattered light reflected by the specimen, and an electrical detector to detect an electrical response (e.g., current or voltage) generated by the specimen in response to the stimuli. Such optical measurement system may include an analysis component to calculate the IQE of the specimen based on signals generated by the measurement system.
Often, in the aforementioned optical measurement systems, significant noise may be present in the signals measured or generated by the detectors and specimen. In some cases, the noise is so prevalent that DC sampling the signals may not be possible or may result in erroneous detection. To combat noise, some optical measurement systems employ a dedicated lock-in amplifier to extract signals buried in noise. According to this technique, the intensity, frequency, or phase of the incident light is modulated at a frequency. The dedicated lock-in amplifier receives and mixes the detector signal with a signal with an established phase relationship with the modulation frequency (often referred to as coherent or heterodyne detection). The mixed signal is then passed through a filter to generate essentially the detector signal with reduced noise.
A drawback to such optical measurement systems is how task specific the dedicated lock-in amplifier are designed. This makes it difficult to re-configure the system and apply it towards measurements that do not require or cannot utilize lock-in functionality. An example would be in a system that is required to measure both the EQE and IQE of specimens that can or cannot respond to the frequency of modulation on the stimulating light source.